Glass is used in a wide range of electronic applications because it possesses a number of desirable properties, including transparency and high thermal stability.
An example of an electronics product utilizing glass is a flat-panel display (FPD). FPDs are becoming ubiquitous; they are in homes, in the workplace and most other places, as well. And their use is increasing. See, for example, a series of articles concerning flat panel displays appearing in the Materials Research Society Bulletin for March 1996; Im et al., "Materials for Flat-Panel Displays," p. 27; Moffatt, "Glass Substrates for Flat Panel Displays," pp. 31-34; Hanna et al., "Materials in Active-Matrix Liquid-Crystal Displays," pp. 35-38; Rack et al., "Materials Used in Electroluminescent Displays," pp. 49-58; and Weber et al., "Materials and Manufacturing Issues for Color Plasma Displays," pp. 65-68. These articles are incorporated by reference herein.
One example of a FPD is an active-matrix liquid crystal display (AMLCD). Such displays are expected to soon become the dominant technology for high resolution, dense information-content visual communication systems. Lightweight, rugged, impact-resistant, high resolution AMLCDs for portable systems such as pagers, phones, personal digital assistants and the like are desired. Minimizing the weight of such displays will relax power dissipation requirements since the weight credit can be applied to additional battery capacity. Ruggedness and impact resistance are especially desirable in portable products due to handling considerations.
Glass, which is presently used for most FPD applications, is a relatively high weight, low impact resistance and high cost material. It would be desirable, therefore, to find a low cost, rugged and light weight alternative to glass for use in AMLCDs and other FPDs. Conventional plastics possess the aforementioned properties and can be used to replace glass in a few FPD applications, e.g., passive LCDs. Conventional plastics are not, however, suitable for use as the plates, i.e., substrates, in AMLCDs and certain other FPDs.
In AMLCDs, each pixel has its own switching element for regulating charges in the liquid crystal cell. Thin-film transistors (TFTs) are typically used as the switching element. The TFT, which is fabricated on the substrate, i.e., typically glass in prior art AMLCDs, consists of a gate electrode, a gate dielectric and a semiconductor layer with two electrodes for a source and a drain. The primary route to thin film transistor (TFT) based AMLCD technology is amorphous-silicon (.alpha.-Si). Properties of the gate dielectric and .alpha.-Si are degraded if such materials are deposited much below 250.degree. C. Other FPD applications, such as electroluminescent (EL) displays and plasma display panels (PDPs) require even higher temperature processing steps.
Conventional plastics can typically withstand processing temperatures of up to only about 150.degree. C. High temperature plastics are available, but these plastics are typically not suitably transparent for display applications. Such high temperature plastics are also typically birefringent. Birefringent materials have different refractive indices (optical density) in different co-ordinate directions. Images presented on a display formed from a birefringent material may appear to be somewhat skewed or otherwise imperfect when observed from various viewing perspectives. Thus, displays should preferably exhibit no more than a minimal amount of birefringence. Furthermore, due to the high coefficients of thermal expansion, the dimensional stability of most plastics with repeated temperature cycling is such that it would be difficult to achieve suitable registration and overlay accuracy for TFTs with multiple mask levels as used in AMLCDs. Thus, while conventional plastics can be used as the substrate for some FPD applications utilizing lower processing temperatures, e.g., passive LCDs, they can not be used for FPDs requiring high temperature processing.
It would be desirable, therefore, to replace the glass substrates used in such FPDs with a substrate that is transparent, not birefringent, possesses the low weight, robustness and low cost of plastics and has a high temperature stability not normally associated with such plastics.